Magnetic recording media essentially comprise flexible non-magnetic substrates to which there has been applied, generally on one side, a magnetic coating, which is generally of a very small thickness, usually less than 10 .mu.m. Calendering is necessary in order to compress the magnetic layer and smooth the surface, thus accomplishing a polishing of the surface.
Apparatuses for calendering are known, in which a plurality of rolls, with or without intermediate flexible rolls, are arranged one above the other in roll mills. For example, the monograph by Braginskij Magnetic Tape Production), page 271, discloses calenders having three to nine rolls. A typical roll assembly comprises, for example, five metallic rolls and four flexible rolls, which are arranged one above the other and between which the magnetic web to be calendered is past back and forth through this arrangement, the magnetic surface constantly facing the hard metallic roll. This roll is generally heated, so that the surface of this roll may be at a temperature between 40.degree. and 200.degree. C. The flexible rolls usually comprise a steel core, to which a special paper, cotton, polyamide, polyurethane or a similar material is applied. Problems arise in the case of this arrangement due to excessive wear of the flexible rolls and due to the bowing of the compression rolls, this bowing being counteracted by a convexity of the roll.
DE 1,561,739 describes a freely idling central roll, arranged around which are a plurality of supporting rolls, which may or may not be adjustable with respect to one another, and the magnetic web is wrapped around the central roll. It is intended in this way to make setting of the contact pressure easier and more uniform.
DE 3,416,211 describes a similar arrangement, only in the case of this calender the central roll is fixedly mounted, as are the supporting rolls. In this case, the supporting rolls are designed as bowing-controlling rolls, on the roll surfaces of which supporting elements loaded by pressure transducers are mounted in distributed arrangement over the length, and a control device is able to set individually or in groups the pressures to be fed to the pressure transducers. In this arrangement, the central roll preferably has a larger diameter than the supporting rolls. A major disadvantage of the arrangement just described is that, to clean the central roll, the calender has to be stopped and the web unthreaded, so continuous operation is not possible.